Model systems were developed at Cornell University for the assessment of insect resistance management (IRM) strategies for Bt crops and insecticides. The major components for Bt crops were (1) Bt broccoli plants expressing a Cry1A, Cry1C, or both Cry1A+Cry1C proteins; (2) diamondback moth (Plutella xylostella) populations resistant to Cry1A, Cry1C, or both Cry1A+Cry1C proteins. The major components for insecticides were P. xylostella populations resistant to Bt formulation, spinosad, and indoxacarb. Greenhouse cage study methods were used for the assessment of IRM strategies with 24-26 generations of P. xylostella on Bt plants or 9 generations of P. xylostella on insecticides. The results showed that (1) resistance to pyramided two-gene Bt plants expressing two dissimilar Bt genes was significantly delayed as compared to single-gene plants deployed in mosaics; (2) concurrent use of Bt plants expressing a single and two Bt genes will select for resistance to two-gene plants more rapidly than the use of two-gene plants alone; and (3) insecticide rotation every insect generation was better for IRM than if the insecticide was rotated every third generation or if the three insecticides were applied as a mosaic.
